Volume reduction of aqueous waste by evaporative crystallization of burkeite and sodium salts

ABSTRACT

A method of treating aqueous salt solutions to provide a solution suitable for vitrification to a stable glass matrix for long term storage is described. In particular, salt solutions composed of aqueous nuclear waste materials are suitable for treatment by the described method. Specifically, salt solutions which have a sulfate to sodium mole ratio that does not permit easy vitrification into stable glasses may be treated by the present invention. The present method decreases the volume of vitrified glass.

This application claims priority under 35 U.S.C. § 119 to U.S.Provisional Patent Application No. 60/276,920 filed on Mar. 20, 2001,and to a U.S. Provisional Patent Application No. 60/365,171, entitled“Sulfate Removal from Hanford Aqueous Waste by Burkeite EvaporativeCrystallization” filed on Mar. 19, 2002, the disclosures of which arehereby incorporated by reference in their entireties.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to methods of treating aqueous salt solutions toprovide a solution suitable for vitrification to a stable glass matrixfor long term storage. The present process involves removal bycrystallization of burkeite, a congruent double salt with the chemicalformula, Na₆(SO₄)₂(CO₃), and sodium complex salts from aqueous saltsolutions. In particular, salt solutions composed of aqueous nuclearwaste materials are suitable for treatment by the invention.Specifically, salt solutions which have a sulfate to sodium mole ratiothat does not permit easy vitrification into stable glasses may betreated by the present invention.

2. Discussion of the Related Art

Large volumes of radioactive aqueous waste have been generated duringplutonium production and other nuclear operations. These wastes arestored in storage tanks at various locations, for instance, the U.S.Department of Energy's (“DOE”) Hanford, Washington site. At present, theDOE Hanford site stores approximately 50 million gallons of radioactiveaqueous waste. Major components in the waste include various soluble andinsoluble compounds including salts of sodium, aluminum, and phosphorousincluding some of the sulfate, nitrate, nitrite, oxide, carbonate, andhydroxide salts of those metals. In these complex mixtures, thesolubility of a specific salt will obviously depend on such factors astemperature and pH of the mixture. Radioactive components in the aqueouswaste include strontium, cesium, technetium, cobalt, uranium, andplutonium.

Present plans for disposal of these waste solutions call for thevitrification of the liquid wastes into glass matrices suitable forstable long-term storage. The vitrification process requires that thecomposition of the waste solution fall within certain parameters toensure production of a stable glass matrix without formation ofundesirable salt phases. One such parameter is the sulfate to sodiummole ratio, which if in excess of 0.01 SO₄ ⁻²/Na⁺, the sodium sulfatepresent may exceed the glass solubility limit, and form a non-misciblesalt phase during the vitrification process. These non-miscible sulfatesalts can corrode the refractory lining of ceramic-lined vitrificationunits, and thus significantly reduce the operating life of thevitrification equipment. Furthermore, in sufficient quantity, theaccumulation of non-miscible sulfate salts provides an electricallypreferred circuit in Joule-heated vitrification melters, thus bypassingthe molten glass matrix and significantly reducing the waste processingrate. Presently, the sulfate to sodium mole ratio in numerous DOEHanford aqueous waste tanks exceeds the glass solubility limit.

Sodium sulfate can be a major contaminant to the vitrification process.At DOE Hanford, for instance, sodium sulfate accounts for approximately3.3% of the total sodium salts. Under the current processing approach tomeet the sulfate vitrification specification, additives, such as sodiumhydroxide, must be added to the waste to reduce sodium sulfate to lessthan 2% of the total sodium salts. By this method, the amount ofvitrification glass and processing time are increased proportionally tothe amount of sodium hydroxide added to the waste.

Sodium sulfate has limited solubility in glass. In equilibratedsolutions, sodium sulfate solubility in glass is 0.5 wt % SO₃ at a Na₂Oconcentration of 14 wt. %. See VSL-00R3630-1, Summary of PreliminaryResults on Enhanced Sulfate Incorporation During Vitrification of LAWFeeds, I. L. Pegg, et al., Vitreous State Laboratory, The CatholicUniversity of America, Washington D.C. This corresponds to a SO₄ ⁻²/Na⁺mole ratio of 0.013. Experimental testing of glass mixtures indicatesthat a molten sulfate salt layer forms well before the glass melt issaturated with sodium sulfate. The sulfate concentration at which a saltphase forms has not been quantified, but thick salt layers have beenobserved in test melters at less than three-quarters of sulfate glasssaturation.

Presence of a molten sulfate salt phase in the melter is highlyundesirable for several reasons. Feeding slurry onto a molten sulfatelayer could cause over-pressurization or steam explosion in the melter.See DOE/RL-98-01, Rev. 3 Sulfate Mitigation for Hanford Tank LowActivity Waste Vitrification, Technology Needs/Opportunities StatementRL-WT101. Molten sulfate salts are more corrosive than the glass melt,can penetrate the refractory of the melter, and cause electricalshorting and corrosion of the melter components. These molten sulfatephases also tend to sequester a variety of hazardous and radioactiveelements, including, for example, cesium and chromium. Furthermore, thesulfate salts are highly soluble in water, which renders the glassproduct unacceptable for long-term storage.

In addition, sulfate may be reduced to sulfur dioxide in the melter,which may be absorbed in a caustic scrubber, to be recycled to themelter, or it may escape the system altogether and become an atmosphericpollutant.

Clearly, the presence of sulfate in the vitrification system isdetrimental to safety, the equipment, the resulting glass, and theenvironment. At the current sulfate feed concentrations of DOE Hanfordaqueous wastes, sodium sulfate will frequently be above the saturationconcentration in the glass and thus a molten salt phase is expected tooccur. The DOE Hanford aqueous waste contains approximately 4,800 metrictons of sodium sulfate.

The current processing approach to adjust the sulfate/sodium ratio tobelow 0.010 SO₄ ⁻²Na⁺ is to add additives, such as sodium hydroxide, tothe aqueous waste until the desired ratio is reached. By this method,the resulting amount of glass and processing time, are both increased byapproximately 86% over a non-sulfate containing vitrification feedstock.Additionally, the addition of the required amounts of sodium hydroxidesignificantly increases the treatment cost.

Others have tried unsuccessfully to adjust the sulfate/sodium ratio bydecreasing the sulfate concentration through removal of sulfate byevaporation and selective precipitation of sulfate, and concluded thatevaporation is not a viable option for removing sulfate. See PNWD-3036;BNFL-RPT-018, Removal of Sulfate Ion From AN-107 by Evaporation, G. J.Lumetta, et al., Pacific Northwest National Lab., Richland, Wash.

Clearly there is a need for a method to process radioactive aqueouswaste with high sulfate/sodium ratios generated by nuclear activities topermit such waste to be vitrified into stable glass matrices. Ideallysuch a sulfate/sodium ratio adjustment process would decrease chemicalcosts, processing expense, and volume of glass matrix produced, whileremoving both sulfate and sodium containing compounds in a stable form.

SUMMARY OF THE INVENTION

The present invention meets the above-stated needs and overcomes thedrawbacks of the current processing approach by removing sulfate, in theform of stable burkeite, to adjust the sulfate/sodium ratio of aqueousradioactive waste solutions to permit vitrification without increasingglass volume with additives or forming non-miscible sulfate salts. Inparticular, the present invention accomplishes these objectives byproviding a process for the removal of burkeite from a salt solutioncontaining aluminum ions, nitrite ions, nitrate ions, sodium ions,calcium ions, and sulfate ions. The inventive process involves eitheradding water and heating, or adding alkali to form a solution containingalkali soluble compounds, followed by optionally filtering the solutionto remove any undissolved solids and to produce a solution essentiallyfree of solids. The undissolved solids are typically wastes which arehigher in radioactivity than the solution. Excess water is thenevaporated from the solution essentially free of solids to produce asaturated solution from which burkeite is precipitated, by evaporativecrystallization, and separated.

The present inventive process may reduce sulfate by an order ofmagnitude, well below the glass solubility limit, and prevent a moltensalt phase from forming in the melter.

The present inventive process for the removal of burkeite from a saltsolution containing aluminum ions, sodium ions, calcium ions, andsulfate ions involves adding alkali to a salt solution comprising saltscontaining aluminum ions, nitrite ions, nitrate ions, carbonate ions,sodium ions, calcium ions, and sulfate ions to form a solutioncontaining alkali soluble compounds, then optionally filtering thesolution to remove any undissolved solids and to produce a solutionessentially free of solids. Excess water is then evaporated from thissolution to produce a saturated solution. Burkeite is then precipitated,preferably by evaporative crystallization, from the saturated solution,and finally separated from the saturated solution.

Additional processing measures which may also be applied to the saltsolution, prior to the evaporating step, include oxidizing any organiccompounds present in the salt solution, precipitating any strontiumcontaining compounds present in the salt solution, and removing anyradionuclides present in the salt solution. Among the radionuclideswhich may be removed are cesium and technetium, typically ion exchangemay be used.

The salt solutions processed by the present inventive method may becomposed of salts which contain at least one negatively charged memberselected from the group consisting of, for example, carbonate, chromate,fluoride, hydroxide, nitrite, nitrate, oxide, silicate, sulfate, andphosphate, and one positively charged member selected from the groupconsisting of, for instance, aluminum, barium, calcium, cesium, iron,manganese, nickel, sodium, strontium, technetium, plutonium, potassium,uranium, and zirconium.

After the burkeite is separated from the saturated solution, it may besolubilized and reprecipitated by addition of either or both of calciumcarbonate and barium sulfate, using conventional technology.

Typically alkali may be added only until any amphoteric aluminumcompounds present in the salt solution are solubilized. Alkali compoundsthat may be used for dissolving aluminum solids include, for example,sodium hydroxide, potassium hydroxide, calcium hydroxide, and otherhydroxide containing compounds.

Evaporative crystallization may be utilized to precipitate burkeite fromthe saturated solution. This evaporative crystallization may occurwithin a forced circulation evaporator. It is preferable that thesaturated solution be essentially free of solids prior to undergoingevaporative crystallization.

After separation of the precipitated burkeite the saturated solution maybe vitrified according to methods known in the art. The saturatedsolution may be vitrified alone, or it may be combined with other saltsolutions then vitrified. In some cases, additional Nat may need to beadded to reach a suitable SO₄ ²/Na⁺ mole ratio to produce a glass matrixsuitable for long-term storage.

Another embodiment of the present invention is a process for the removalof burkeite from a radioactive salt solution by treating the radioactivesalt solution to produce a saturated salt solution essentially free ofsolids. Burkeite is then precipitated from the saturated salt solution,and finally separated from the saturated salt solution.

Additional processing steps that may occur prior to precipitatingburkeite include adding alkali, separating alkali insoluble compounds,oxidizing organic compounds, precipitating strontium containingcompounds, removing radionuclides, and then evaporating excess water.

Yet another embodiment of the invention is a method to reduce thesulfate concentration of a salt solution by adding alkali to a saltsolution comprising salts containing aluminum ions, nitrite ions,nitrate ions, carbonate ions, sodium ions, calcium ions, and sulfateions to form a solution containing alkali soluble compounds. Optionally,this solution may then be filtered to remove any undissolved solids andto produce a solution essentially free of solids. The undissolved solidsare typically higher in radioactivity than the solution. Excess watermay be evaporated from the solution essentially free of solids toproduce a saturated solution. Sulfate-rich material may be precipitatedfrom the saturated solution, and then the precipitated sulfate-richmaterial may be separated from the saturated solution.

A different embodiment of the present invention may be a process for theremoval of burkeite and sodium-containing complex salts from a saltsolution containing aluminum ions, nitrite ions, nitrate ions, carbonateions, sodium ions, calcium ions, fluoride ions and sulfate ions byadding hydroxide to a salt solution comprising salts containing aluminumions, nitrite ions, nitrate ions, carbonate ions, sodium ions, calciumions, fluoride ions, and sulfate ions to solubilize aluminum containingcompounds thereby to form a solution containing soluble aluminumcompounds from the salt solution. Optionally, any undissolved solidmaterial may be filtered off from the solution containing solublealuminum compounds to produce a solution essentially free of solids.Excess water may be evaporated from the solution essentially free ofsolids to produce a saturated solution. Burkeite and sodium-containingcomplex salts may be precipitated from the saturated solution, and thenthe precipitated burkeite and sodium-containing complex salts may beseparated from the saturated solution.

This embodiment may further include: one or more of the following stepsas known in the art, prior to the evaporating step: oxidizing anyorganic compounds present in the salt solution, precipitating anystrontium containing compounds present in the salt solution, andremoving any radionuclides present in the salt solution.

The radionuclides may be cesium and technetium, and may be removed byion exchange, or other conventional ion separation techniques.

The saturated solution may be vitrified after separation of theprecipitated burkeite and sodium-containing salts to obtain a glassmatrix suitable for long-term storage. The vitrification may beaccomplished by conventional processes.

A further embodiment of the present invention involves theco-crystallization and removal of sodium-containing complex salts, suchas, the carbonate, sulfate, and fluoride salts of sodium along with theburkeite. Additional sodium carbonate salts include, for example, theanhydrate, and the mono-, hepta-, and deca-hydrates. Further sodiumsulfate compounds include sesquiburkeite (Na₂SO₄.Na₂CO₃) and anhydroussodium sulfate. Sodium fluoride salts may include sodium fluoride, andtrisodium fluoride sulfate.

These sodium-containing complex salts may be crystallized and removedduring the burkeite evaporative crystallization process. The removal ofthese sodium salts has the additional benefits of further reducing thevolume of radioactive glass, avoiding increased corrosive effects, andincreasing glass stability.

Additional features, advantages, and embodiments of the invention may beset forth or apparent from consideration of the following detaileddescription, drawings, and claims. Moreover, it is to be understood thatboth the foregoing summary of the invention and the following detaileddescription are exemplary and intended to provide further explanationwithout limiting the scope of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this specification, illustrate preferred embodiments of theinvention and together with the detailed description serve to explainthe principles of the invention. In the drawings:

FIG. 1—Solubility diagram of sodium sulfate and sodium carbonate withburkeite solubility as a function of temperature and concentration ofsodium sulfate and sodium carbonate graphed thereon.

FIG. 2—Solubility diagram of the ternary system—sodium sulfate, sodiumcarbonate, and sodium nitrate as a function of temperature andconcentration.

FIG. 3—Block flow diagram of one embodiment of the inventive process asapplied to DOE Hanford waste tank AZ-102.

FIG. 4—Block flow diagram of one embodiment of the inventive processapplied to DOE Hanford waste tank AN-102.

DETAILED DESCRIPTION OF THE INVENTION

The present invention decreases the volume of aqueous waste by removingboth sulfate and sodium-containing complex salts through an evaporativecrystallization process. Specifically, the present invention reducessulfate in aqueous waste by either adding water and heating, or addingalkali to sulfate-rich vitrification feedstocks, evaporating excesswater, precipitating and separating burkeite, a congruent double saltwith the chemical formula, Na₆(SO₄)₂(CO₃). The precipitation of burkeiteis accomplished by evaporative crystallization. This reduction insulfate either brings the SO₄ ⁻²/Na⁺ mole ratio of the feedstock belowthe present 0.010 SO₄ ⁻²/Na⁺ glass formulation limit, or greatly reducesthe amount of additional glass additives required to meet the ratio. Inturn, this process reduces the volume of material to be treated and theresulting amount of glass produced.

Sulfate-rich vitrification feedstocks typically may have sulfateconcentrated in the crust, supernatant, and/or sludge layer. Thesulfate-rich phase may be separated by decantation, filtration or otherseparation techniques known in the art from any phases meeting thesulfate to sodium specification.

The sulfate-rich feedstock solution may be treated by adding alkali,typically sodium hydroxide, to dissolve alkali soluble compounds, andseparating alkali insoluble materials. This process step may be carriedout in existing storage tanks, or in a separate operation location. Thisstep is also known as caustic leaching, or enhanced sludge washing. Ofparticular interest in the present invention is to add sufficient sodiumhydroxide to cause dissolution of solid amphoteric aluminum compounds,such as aluminum trihydroxide and/or dawsonite, NaAlCO₃(OH)₂.

Alternatively, and dependent on the exact chemical composition of thefeedstock, addition of only water and heating will dissolve theinsoluble materials, including amphoteric aluminum solids. Theavailability of hydroxide ions in the waste is one factor considered inwhether addition of additives, such as NaOH, is needed to dissolve anyamphoteric aluminum solids, or whether such dissolution will occur withthe addition of water only.

Optionally, the mixture may be filtered to remove any undissolvedsolids. Such undissolved solids are typically higher in radioactivitythan the solution.

The high alkali sulfate-rich solution is then saturated by preheatingand evaporating the solution to the burkeite solubility limit. Theevaporation process may be carried out in either a batch-wise or acontinuous manner.

This saturated solution may be further evaporated in an evaporativecrystallization process so that the solution exceeds the burkeitesolubility limit, and burkeite crystallizes from solution. Crystalseeding with sodium sulfate and/or burkeite may be used to introducecrystal nuclei and increase the sulfate yield and selectivity.Additional sodium-containing complex salts of sulfate, carbonate, andfluoride may be co-crystallized with burkeite to further reduce theamount of sodium fed to the vitrification process.

The evaporative crystallization process of the present invention may becarried out in a conventional forced-circulation evaporator designed forevaporative crystallization, such as that available from Swenson ProcessEquipment of Monee, Ill.

During the evaporative crystallization process, the temperature of thesolution may be between about 50° C. and about 150° C. with the pressurebetween about 0.2 atmosphere to about 2 atmospheres. Preferably thepressure is between about 0.1 atmosphere to about 1 atmosphere. The mostpreferred evaporative crystallization conditions employ a solutiontemperature between about 80° C. and about 120° C. at pressure of about0.2 atmosphere.

The precipitated solid burkeite, and, if present, other precipitatedcomplex sodium-containing salts, are separated from the aqueous phaseand isolated from the vitrification process. Accordingly the inventiveprocess may be used to remove other complex salts such assodium-containing complex salts, such as, the carbonate, sulfate, andfluoride salts of sodium along with the burkeite. Additionalsodium-containing salts include, for example, the anhydrate, and themono-, hepta-, and deca-hydrates of sodium carbonate, further sodiumsulfate compounds include sesquiburkeite (Na₂SO₄.Na₂CO₃) and anhydroussodium sulfate, and sodium fluoride salts may include sodium fluoride,and trisodium fluoride sulfate. The aqueous phase, thus depleted insulfate, and perhaps sodium, is fed to a vitrification process. Aconventional vitrification process is described in U.S. Pat. No.6,258,994, the disclosure of which is hereby incorporated by referencein its entirety.

The present inventive methods utilize the thermodynamically favored,naturally occurring congruent double salt of sodium sulfate and sodiumcarbonate, burkeite, to selectively reduce the quantity of sulfate fromaqueous phase radioactive waste. By treating the waste solution prior toevaporative crystallization of burkeite, precipitation of unwantedcompounds that do not decrease the sulfate concentration is avoided.Following crystallization, burkeite solids are separated from theaqueous liquor which now meets or is closer to the sulfate/sodium ratiotarget for vitrification.

Burkeite has a 2:1 sodium sulfate (Na₂SO₄) to sodium carbonate (Na₂CO₃)mole ratio and a 71:29 sodium sulfate to sodium carbonate mass ratio.Because of this mass ratio, evaporative crystallization of solidburkeite from the aqueous phase preferentially removes sulfate from theaqueous phase.

Burkeite, and its component salts, anhydrous sodium carbonate and sodiumsulfate, all exhibit retrograde solubility, that is, decreasingsolubility with increasing temperature. A solubility diagram of thesodium sulfate—sodium carbonate system is shown in FIG. 1.

The burkeite evaporative crystallization process is outlined on thesodium sulfate-sodium carbonate solubility diagram in FIG. 1. Uponevaporation of the sulfate-rich carbonate solution (Point A), theconcentrations of sodium sulfate and sodium carbonate increaseproportionally up to the burkeite solubility limit at atmosphericpressure (Point B). During burkeite crystallization, the aqueous phaseis reduced in sulfate mass in a 71:29 mass ratio to carbonate, along theburkeite solubility curve, to the burkeite—sodium carbonate invariant(Point C). Upon further evaporation, burkeite and sodium carbonateco-crystallize from solution.

The effect of additional soluble sodium salts on burkeite solubility isshown in FIG. 2. At saturation, burkeite crystallizes from an aqueoussolution having the initial concentration depicted at point A. Asburkeite crystallizes from the aqueous solution, the aqueous phasebecomes depleted in both sodium sulfate and sodium carbonate until thesodium carbonate solubility limit is reached at invariant point B.Burkeite and sodium carbonate then co-crystallize along the univariantline to the sodium nitrate solubility limit, point C. At this point,evaporation is terminated, and both burkeite and sodium carbonate areseparated from the aqueous solution.

The present inventive process may be utilized on salt solutions ofvarying complexity and composition. The co-crystallization of bothburkeite and other sodium-containing complex salts from the saltsolution decreases the concentration of both sulfate and sodium ions inthe salt solution with concurrent reduction in the volume of the saltsolution. The volume reduction may also be reflected in the smallervolume of glass produced by the vitrification of the remaining saltsolution.

EXAMPLE 1

Application of the inventive process for sulfate removal to aqueouswaste from DOE Hanford Waste Tank AZ-102 is graphically depicted in FIG.3. Theoretical material mass balances for the various steps arepresented in Table 1. In this example, 3,748 m³ of radioactive aqueouswaste containing 96,085 kilograms of sodium sulfate with an initialsulfate to sodium mole ratio of 0.069, and 4.1% solids would be treatedby the inventive process.

Sodium hydroxide (approximately 640,000 kilograms of a 50% solution)would be added to dissolve amphoteric aluminum compounds and alkalisoluble sodium compounds. Insoluble aluminum trihydroxide is convertedto soluble aluminum tetrahydroxide ions by reaction with sodiumhydroxide, reducing the total solids from 4.1 to 2.1%.

The next step would be to decant the aqueous phase to separate residualinsoluble solids from the solution. Residual insoluble compounds wouldinclude, for example, oxides of iron, manganese, nickel, and zirconium,along with radioactive compounds of uranium, strontium, and plutonium.

The decanted solution would then undergo evaporative crystallization ofburkeite. The evaporator would be seeded with burkeite and/or sodiumsulfate crystals to provide nucleation sites for the crystallization. Inthis example, at a temperature of 52° C. and 0.2 atmospheres,approximately 1,845,400 kilograms of water would be evaporated from theradioactive waste.

The evaporative crystallizer product would contain approximately 68,607kilograms of sodium sulfate in approximately 94,208 kilograms ofburkeite. This quantity equates to removal of 71.4% of the total sulfatepresent in the AZ-102 waste.

Following burkeite crystallization, the solid phase would bemechanically separated from the aqueous liquor, and isolated from thevitrification plant feedstock. The aqueous phase would then be fed tothe vitrification plant.

In this example, the aqueous phase would have a final sulfate to sodiummole ratio of 0.012. In order to meet the glass formulation constraintof 0.010 mole ratio, approximately 267,600 kilograms of 50% sodiumhydroxide would have to be added to the solution.

The inventive removal process would reduce the net amount of glass wasteproduced from DOE Hanford Tank AZ-102 by a factor of 3.5 relative tountreated waste.

COMPARATIVE EXAMPLE 2

Under the current processing approach to meet the glass formulationconstraint of 0.010 SO₄ ⁻²/Na⁺, sodium hydroxide would be added to theAZ-102 waste. This approach would require approximately 4,661,200kilograms of 50% sodium hydroxide added to the waste. This approachwould increase the total amount of sodium by a factor of 5.9, and themass of glass to be produced by vitrification would increase by the samefactor, both relative to untreated waste.

EXAMPLE 3

Application of the inventive process for sulfate removal to aqueouswaste from DOE Hanford Waste Tank AN-102 is graphically depicted in FIG.4. Theoretical material mass balances are presented in Table 2. In thisexample, 3,981 m³ of radioactive aqueous waste containing 105,401kilograms of sodium sulfate with an initial sulfate to sodium mole ratioof 0.019 and 4.3% solids would be treated by the inventive process.

Due to the composition of the AN-102 waste solution, addition of NaOH isnot required prior to the evaporative crystallization of burkeite. Thus,the sulfate removal process would begin by adding approximately 411,950kilograms of dilution water and heating to 50° C. to dissolve solidsodium compounds. AN-102 solids include both trisodium fluoride sulfateand burkeite.

The next step would be to decant the aqueous phase to separate residualinsoluble solids from the solution. Residual insoluble compoundsinclude, for example, oxides of iron, manganese, nickel, and zirconium,along with radioactive compounds of uranium, strontium, and plutonium,for example.

The decanted solution would then undergo evaporative crystallization toobtain burkeite. The evaporative crystallizer could be seeded withburkeite and/or sodium sulfate crystals to provide nucleation sites forthe crystallization. In this example, approximately 317,590 kilograms ofwater would be evaporated at a temperature of 52° C. and 0.2atmospheres. Water would be evaporated from the solution until theburkeite-sodium carbonate monohydrate invariant of 0.14 is reached.Beyond this point, burkeite and sodium carbonate co-crystallize fromsolution.

The evaporative crystallizer product would contain approximately 33,354kilograms of sodium sulfate in approximately 45,800 kilograms ofburkeite. This quantity equates to removal of approximately 31.7% of thetotal sulfate present in the AN-102 waste.

Following burkeite evaporative crystallization, the solid phase wouldthen be mechanically separated from the aqueous liquor, and isolatedfrom the vitrification plant feedstock. The aqueous phase would then befed to the vitrification plant.

In this example, the aqueous phase would have a final sulfate to sodiummole ratio of 0.013. In order to meet the glass formulation constraintof 0.010 mole ratio, approximately 990,500 kilograms of 50% sodiumhydroxide would be added to the solution.

The inventive removal process reduces the net amount of glass wasteproduced from DOE Hanford Tank AN-102 by 31.7% relative to untreatedwaste.

COMPARATIVE EXAMPLE 4

Under the current processing approach to meet the glass formulationconstraint of 0.010 SO₄ ⁻²/Na⁺ mole ratio, sodium hydroxide would beadded to the AN-102 waste. This approach would require approximately2,746,400 kilograms of 50% sodium hydroxide added to the waste. Thisapproach would increase the total amount of sodium by 86%; the mass ofglass to be produced by vitrification would increase by the same factor,both relative to untreated waste.

EXAMPLE 5

Laboratory scale experiments were run to demonstrate the presentinventive method. X-ray diffraction analysis of the resulting productswas completed in order to confirm the validity of the method.

Table 3 is a material mass balance for the five solutions of theexperiment. The amounts of raw reactants, products, and by-products ofthe five solutions are tabulated therein. Evaporative crystallization ofburkeite was carried out under the conditions specified in Table 3.

All experiments were carried out in a ventilation hood using a 1-literglass reaction vessel with a magnetic stir bar. A heating mantle and acondenser set up were used for heating and collection of water vaporfrom the reaction vessel respectively.

Deionized water was quantitatively measured and added to the reactionvessel. Chemical reagents were then measured and added to the reactionvessel. The solution was agitated and/or heated until all solids weredissolved. Heating and stirring were continued until the appropriateamount of condensed water vapor was collected in a condensate receiver.The hot slurry was filtered through a buchner funnel to prevent impuritycrystallization by cooling. The mass of the filter cake was measured.The filter cake was dried in an oven at 110° C. The mass of crystals wasrecorded and the composition of crystals collected was analyzed by X-raydiffraction (XRD), and are presented in Table 4.

The diffraction data were collected on a Siemens D-500 diffractometerusing a slit combination which provides the greatest resolution. Tofurther maximize resolution the data were collected using a small stepsize (0.02 degrees 2-theta). XRD data collected on a Philips XRD3100X-ray diffractometer equipped with a copper x-ray tube (energized at40KV and 35 ma), a graphite monochrometer and a theta compensationvariable slit were compared with those from the Siemens D500diffractometer. The samples were scanned from 4 to 64 degrees 2-theta at1 degree per minute.

Solution A was a mixture of water, sodium carbonate, and sodium sulfateonly. Sodium carbonate is the main impurity to burkeite crystallization,burkeite will form only if the sodium sulfate to sodium carbonate ratiois greater than 0.14. Otherwise, burkeite and sodium carbonateco-crystallize with low sulfate yield.

Solution B included the chemicals in Solution A with the addition ofsodium nitrate and sodium nitrite, the main components of DOE Hanfordaqueous waste. A double salt of sodium sulfate and sodium nitrate(darapskite) is theoretically possible at high nitrate concentrations.

Solution C included the chemicals in Solution B plus sodium hydroxideand aluminum hydroxide. Aluminum sludge is a large component of DOEHanford waste. Aluminum solubility is exponentially proportional to pH.Sodium hydroxide will increase pH to a range where aluminum is highlysoluble, and aluminum sludge will completely dissolved.

Solution D included the chemicals in Solution C plus sodium fluoride. Adouble salt of sodium sulfate and sodium fluoride is possible at highfluoride concentrations. However, at the NaF concentration in AZ-102,Na₃FSO₄ is not expected to form. This method may be used to increasesulfate yield by fortifying the solution with sodium fluoride andprecipitating Na₃FSO₄.

Solution E included all the above chemicals and sodium oxalate. Sodiumoxalate is the principal organic component of DOE Hanford aqueous waste.It is saturated in AZ-102 and most DOE Hanford aqueous waste. Sodiumoxalate is expected to co-crystallize with burkeite at a ratio of 1:10Na₂C₂O₄ to burkeite.

Data Analysis for Solutions A-E

Manual matching of JCPDS (Joint Committee of Powder Diffraction) filepatterns, which are listed below, of the X-ray diffraction for thesamples provided the following results:

Solution A—a mixture of three phases: sodium carbonate, Na₂CO₃(25-0815), sodium carbonate sulfate, Na₄CO₃SO₄ (24-1138), and burkeite,Na₆CO₃(SO₄)₂ (24-1134).

Solution B—a mixture of three phases: Na₂CO₃ (25-0815), possibly sodiumcarbonate sulfate (24-1138), and burkeite (24-1134).

Solution C—a mixture of three phases: Na₂CO₃ (25-0815), possibly sodiumcarbonate sulfate (24-1138), and burkeite (24-1134).

Solution D—a mixture of two phases Na₂CO₃ (25-0815) and burkeite(24-1134).

Solution E—three phases Na₂CO₃ (25-0815), burkeite (24-1134) and sodiumoxalate.

Although the foregoing description is directed to the preferredembodiments of the invention, it is noted that other variations andmodifications will be apparent to those skilled in the art, and may bemade without departing from the spirit or scope of the invention.

What we claim is:
 1. A process for the removal of burkeite from a saltsolution comprising aluminum ions, carbonate ions, fluoride ions, sodiumions, calcium ions, and sulfate ions comprising: adding alkali to a saltsolution comprising salts comprising aluminum ions, carbonate ions,fluoride ions, sodium ions, calcium ions, and sulfate ions to form asolution containing alkali soluble compounds; optionally filtering thesolution to remove any undissolved solids and to produce a solutionessentially free of solids; evaporating excess water from the solutionto produce a saturated solution, wherein during the evaporating step thetemperature of the solution is between about 50° C. and about 150° C.and the pressure of the solution is between about 0.2 atmospheres toabout 2 atmospheres—has been inserted; precipitating burkeite from thesaturated solution; and separating the precipitated burkeite from thesaturated solution.
 2. A process according to claim 1, wherein thepressure of the solution is between about 0.1 atmosphere to about 1atmosphere.
 3. A process according to claim 2, wherein the temperatureof the solution is between about 80° C. and about 120° C. and thepressure of the solution is about 0.2 atmosphere.
 4. A process accordingto claim 1, wherein said evaporating step further comprises:crystallizing and removing carbonate, sulfate, and fluoride salts ofsodium.
 5. A process according to claim 4, wherein said carbonate saltsof sodium comprise the anhydrate, monohydrate, heptahydrate, anddecahydrate of sodium carbonate.
 6. A process according to claim 4,wherein said sulfate salts of sodium comprise sesquiburkeite andanhydrous sodium sulfate.
 7. A process according to claim 4, whereinsaid fluoride salts of sodium comprise sodium fluoride and trisodiumfluoride sulfate.
 8. A process according to claim 1, wherein saidprocess further comprises one or more of the following, prior to theevaporating step: oxidizing any organic compounds present in the saltsolution; precipitating any strontium containing compounds present inthe salt solution; and removing any radionuclides present in the saltsolution.
 9. A process according to claim 8, wherein said radionuclidescomprise at least one member selected from the group consisting ofuranium, plutonium, cobalt, strontium, cesium and technetium.
 10. Aprocess according to claim 1, wherein the salt solution furthercomprises salts comprising: at least one member selected from the groupconsisting of carbonate, chromate, fluoride, hydroxide, nitrite,nitrate, oxide, silicate, and phosphate, and at least one memberselected from the group consisting of aluminum, barium, calcium, cesium,iron, manganese, nickel, sodium, strontium, technetium, plutonium,potassium, uranium, and zirconium.
 11. A process according to claim 1,wherein said addition of alkali solubilizes any amphoteric aluminumcompounds present in the salt solution.
 12. A process according to claim1, wherein the salt solution comprises a radioactive waste solution. 13.A process according to claim 1, wherein precipitating burkeite comprisescrystallizing burkeite from the saturated solution by evaporativecrystallization.
 14. A process according to claim 13, wherein theevaporative crystallization occurs within a forced circulationevaporator.
 15. A process according to claim 1, further comprising:vitrifying the saturated solution after separation of the precipitatedburkeite.
 16. A process for the removal of burkeite andsodium-containing complex salts from a salt solution comprising aluminumions, sodium ions, carbonate ions, calcium ions, fluoride ions andsulfate ions comprising: adding either hydroxide or water to a saltsolution comprising salts comprising aluminum ions, sodium ions,carbonate ions, calcium ions, fluoride ions and sulfate ions tosolubilize aluminum containing compounds thereby to form a solutioncontaining soluble aluminum compounds from the salt solution; optionallyfiltering off any undissolved solid material from the solutioncontaining soluble aluminum compounds to produce a solution essentiallyfree of solids; evaporating excess water from the solution essentiallyfree of solids to produce a saturated solution, wherein during theevaporating step the temperature of the solution is between about 50° C.and about 150° C. and the pressure of the solution is between about 0.2atmospheres to about 2 atmospheres—has been inserted; precipitatingburrkeite and sodium-containing complex salts from the saturatedsolution; and separating the precipitated burkeite and sodium-containingcomplex salts from the saturated solution.
 17. A process according toclaim 16, wherein the pressure of the solution is between about 0.1atmosphere to about 1 atmosphere.
 18. A process according to claim 17,wherein the temperature of the solution is between about 80° C. andabout 120° C. and the pressure is about 0.2 atmosphere.
 19. A processaccording to claim 16, wherein said sodium-containing complex saltscomprise carbonate, sulfate, and fluoride salts of sodium.
 20. A processaccording to claim 19, wherein said carbonate salts of sodium comprisethe anhydrate, monohydrate, heptahydrate, and decahydrate of sodiumcarbonate.
 21. A process according to claim 19, wherein said sulfatesalts of sodium comprise sesquiburkeite and anhydrous sodium sulfate.22. A process according to claim 19, wherein said fluoride salts ofsodium comprise sodium fluoride and trisodium fluoride sulfate.
 23. Aprocess according to claim 16, wherein said process further comprisesone or more of the following, prior to the evaporating step: oxidizingany organic compounds present in the salt solution; precipitating anystrontium containing compounds present in the salt solution; andremoving any radionuclides present in the salt solution.
 24. A processaccording to claim 23, wherein said radionuclides comprise at least onemember selected from the group consisting of uranium, plutonium, cobalt,strontium, cesium and technetium.
 25. A process according to claim 23,wherein the salt solution further comprises salts comprising: at leastone member selected from the group consisting of carbonate, chromate,fluoride, hydroxide, nitrite, nitrate, oxide, silicate, and phosphate,and at least one member selected from the group consisting of aluminum,barium, calcium, cesium, iron, manganese, nickel, sodium, strontium,technetium, plutonium, potassium, uranium and zirconium.
 26. A processaccording to claim 23, wherein the salt solution comprises a radioactivewaste solution.
 27. A process according to claim 23, further comprising:vitrifying the saturated solution after separation of the precipitatedburkeite, and sodium-containing complex salts.